1. Field of the Invention
The present invention generally relates to a belt driving control device configured to control a belt looped over a plurality of supporting rollers, a belt device having the belt driving control device, an image forming apparatus having the belt device such as a digital multifunction apparatus that includes a combination of functions such as a copying apparatus, a printer, facsimile, or a belt device, a method for controlling the drive of a belt conveyed in the belt driving control device, the belt device, or the image forming apparatus, a computer program for causing a computer to execute the belt driving control method, and a recording medium having the computer program for causing a computer to execute the method for controlling the drive of a belt in the belt driving control device.
2. Description of the Related Art
An image forming apparatus including various belts including a photoreceptor belt, an intermediate transfer belt, a sheet transfer conveyor belt, etc. is generally known to a person skilled in the art as one example of apparatuses utilizing belts. A high degree of accuracy in controlling the drive of the belt is a prerequisite for this type of image forming apparatus in order to insure high image quality. Below, one example of a tandem type electrophotographic image forming apparatus utilizing an intermediate transfer system will be described with reference to FIG. 1.
In the image forming apparatus as shown in FIG. 1, for example, four image forming units 18Y, 18M, 18C, and 18K that form corresponding homochromatic images in colors of yellow (Y), magenta (M), cyan (C), and black (K) are sequentially arranged along the direction of travel of a recording sheet that is being conveyed. Electrostatic latent images formed on surfaces of photoconductor drums 40Y, 40M, 40C, and 40K are then developed by exposure of laser from a laser exposure unit 21 at the corresponding image forming units 18Y, 18M, 18C, and 18K to form toner images (perceivable images). Subsequently, the homochromatic images formed on the surfaces of corresponding photoconductor drums 40Y, 40M, 40C, and 40K of the image forming units 18Y, 18M, 18C, and 18K are temporarily transferred on an intermediate transfer belt such that the homochromatic images are sequentially superimposed. Thereafter, toner of the superimposed images are fused and pressed by a fixation device 25, thereby forming color images fixated on the recording sheet.
In such an image forming apparatus, failure to maintain the travelling velocity of the recording sheet; that is, the travelling velocity of the intermediate transfer belt 10 at a constant value, results in color shifts. Such color shifts result from relative shifts in transferring positions of the homochromatic images that are alternately superimposed on the recording sheet. The color shifts may result in blurring fine line images formed by superimposing images of plural colors or white dot defects around profiles of black character images in the background image formed by superimposing images of plural colors.
In the image forming apparatus, including the aforementioned tandem type image forming apparatus, which utilizes a belt as a recording material transfer member for transferring a recording material or an image carrier such as a photoconductor or an intermediate transfer member to carry images transferred on the recording material, failure to maintain the travelling velocity of the belt at a constant value may result in banding. The banding indicates image density heterogeneity that results from fluctuations in the travelling velocity of the belt while images are being transferred on the recording material.
Specifically, a portion of the image transferred on the intermediate transfer belt 10 when the travelling velocity of the belt is relatively fast has a profile extended in a circumferential direction (i.e., travelling direction) of the belt whereas a portion of the image transferred on the intermediate transfer belt 10 when the travelling velocity of the belt is relatively slow has a profile shrunk in the circumferential direction of the belt, in comparison to the original profile of the image. The extended portion of the image has low density while the shrunk portion has high density.
As a result, the image density heterogeneity in the circumferential direction of the belt or banding is observed. The banding is significantly perceived with the naked eye when pale homochromatic images are formed.
Accordingly, in order to prevent the color shifts, banding, or the like, highly accurate driving control may be required for moving endless belts including the photoconductor belt, the intermediate transfer belt, a transfer conveyor belt, and the like at a constant travelling velocity. There is a technology for obtaining highly accurate driving control of the belt known to a person skilled in the art, in which a rotational velocity of a driving roller driving a belt is controlled at a constant value. In this method, the rotational velocity of the driving roller is maintained at a constant value by stabilizing a rotational angular velocity of a motor of a driving source, or a rotational angular velocity of a gear that transmits rotational driving force generated by the motor to the driving roller.
However, with the technology described above, even though the rotational angular velocity of the driving roller is maintained at a constant value, the travelling velocity of the belt may not always be kept at a constant value. This phenomenon is particularly significant when the thickness of the belt varies along the circumferential direction of the belt.
Japanese Patent Application Laid-Open No. 2006-106642 discloses a technology designed to minimize occurrences of such a phenomenon. In the disclosed technology, in order to form images while obtaining the amplitudes and phases of AC components of a rotational angular velocity and a rotational angular displacement of the belt having frequencies corresponding to fluctuation of the thickness of the belt in the circumferential direction, a driving signal output from a motor is converted into a rotational angular velocity of a driven roller. Subsequently, the driving signal output from the motor and the driving signal input to the motor is compared at a comparator to obtain a fluctuation component derived from a thickness fluctuation of the belt for one cycle rotation. The belt for one cycle rotation or one rotation hereinafter implies an entire length of the circular endless belt. Thereafter, a periodic fluctuation sampling section records the fluctuation component that results from the thickness fluctuation of the belt for one rotation on a memory. A fluctuation amplitude and phase detective section detects amplitudes and phases of a belt rotational component from the fluctuation component for one rotation of the belt recorded on the memory.
Note that there are also well known technologies in which the travelling velocity of the belt is controlled based on the fluctuation obtained by detecting the thickness of the belt in the circumferential direction (see Japanese Patent Application Laid-Open No. 2006-23403, Japanese Patent Application Laid-Open No. 2002-72816, Japanese Patent No. 2754582, and Japanese Patent Application Laid-Open No. 2004-20236).
In the disclosed technology of Japanese Patent Application Laid-Open No. 2006-106642, the driving signal output from the motor is compared with the driving signal input to the motor at a comparator so as to obtain a fluctuation component derived from the thickness fluctuation of the belt obtained for one rotation thereof. That is, in the technology of Japanese Patent Application Laid-Open No. 2006-106642, in order to obtain the fluctuation component and conduct a predetermined control on the motor based on the obtained fluctuation component, data on the thickness may be required for the entire length of the endless belt obtained from one rotation.
Specifically, according to the technologies of the related art, the belt needs to be driven for one rotation in order to sample the fluctuation component of the belt thickness (i.e., thickness deviation). However, with such technologies, even though printing is finished before the belt has made one rotation, the belt may still have to make one complete rotation only to sample data on the thickness deviations of the belt. Thus, it is inefficient to drive the belt only for obtaining data on the thickness deviation of the belt, because the life span of the entire image forming apparatus is reduced (trade-off relationship) for cancelling out the thickness deviation of the belt and improving driving control of the belt.
Thus, attempts have been made to correct deviations of the belt thickness without reducing the life span of the entire image forming apparatus.